1. Field of the Invention
The present invention relates to a radiographic imaging apparatus, a radiographic imaging system, and a radiographic imaging method for radiographing a radiographic image of a subject by using a radiographic image detector.
2. Description of the Related Art
Recently, in the field of radiography, for example, X-ray imaging, instead of an X-ray film or an imaging plate (IP), X-ray image detection devices (hereinafter referred to as electronic cassettes) using a flat panel detector (hereinafter referred to as an FPD), which uses semiconductor elements, as a detector have been widely used. The FPD uses an image sensor formed of a so-called solid-state imaging apparatus which is formed by using the semiconductor elements, and thus the pixels, in which the signal electric charges corresponding to the amounts of incident X-rays are accumulated, are arranged in a matrix. The FPD detects an X-ray image, which indicates image information of a subject, by converting the signal electric charge, which is accumulated for each pixel due to the incidence of the X-rays, into a voltage signal. The X-ray image, which is detected by the FPD, is output as digital image data.
The FPD includes a TFT type, in which pixels including thin film transistors (TFTs) are formed on a glass substrate, and in addition a CMOS type in which pixels are formed on a silicon substrate through a process of manufacturing Complementary Metal-Oxide Semiconductor (CMOS) as described in JP2005-143802A. One of the big differences between the TFT type and the CMOS type is a way of reading the voltage signals. The TFT type FPD is a system in which the signal electric charges accumulated in the pixels are transferred to an integrating amplifier through signal lines and in which the voltage signals corresponding to the signal electric charges integrated by the integrating amplifier are read. The TFT type FPD is a so-called destructive reading system in which the signal electric charges in the pixels are drained through reading. In contrast, the CMOS type FPD is a so-called nondestructive reading system in which the amplifier converting the signal electric charge into the voltage signal is provided in each pixel and the voltage signals corresponding to the signal electric charges are read in a state where the signal electric charges are retained in the pixels.
In the FPD of either of the TFT type or the CMOS type, apparently, there is an upper limit in the amount of accumulated signal electric charge accumulated in the pixel. Thus, if there is an excessively large dosing amount of X-rays incident into the pixel, the pixel is saturated, and so-called overflow occurs. In this case, in the X-ray image, the pixel becomes a distorted black pixel. On the other hand, if there is an excessively small dosing amount of X-rays, that is, if so-called underflow occurs, in the X-ray image, the pixel becomes a distorted white pixel. In X-ray imaging, imaging conditions, such as irradiation time and tube current, determining the dosing amount of rays emitted by the X-ray source are set in advance in accordance with a physical size or an imaging target portion of a subject so as not to cause overflow and underflow.
Further, even when the amount of accumulated signal electric charge accumulated in the pixel does not cause overflow and underflow, if the gain (amplification ratio) of the amplifier at the time of reading the X-ray image is excessively high, this causes overflow, whereas if the gain (amplification ratio) of the amplifier is excessively low, this causes underflow. Hence, in order to obtain an X-ray image with better image quality, by measuring the dosing amount of rays incident into the FPD, the gain (amplification ratio) of the amplifier at the time of reading the X-ray image may be adjusted in accordance with the measured dosing amount. For example, in the CMOS type FPD of JP2005-143802A, using an advantage of a CMOS-type nondestructive reading technique, before reading the X-ray image, based on the voltage signals (pixel values) which are nondestructively read from some pixels in the detection surface, the gains (amplification ratios) of the amplifiers at the time of reading the X-ray image are adjusted.
Specifically, a plurality of determination pixels, which are distributed to be regularly spaced in the detection surface, is provided, and voltage signals (pixel values) are nondestructively read from the plurality of determination pixels during irradiation of X-rays. In addition, among the plurality of determination pixels, a determination pixel of which the pixel value is the maximum is specified as a reference pixel, and the following adjustment is performed: if the pixel value of the reference pixel is large, the gain is decreased; and if the pixel value is small, the gain is increased. As described above, when the gain is adjusted, occurrence of overflow and underflow is reduced.